is valid, cf. [1]}.
Here, \(\nabla _x=\mbox{\rm grad}\) , \(\Delta _x=\nabla _x\cdot \nabla _x\) 
is the Laplace operator and \((\,,)_{\Pi ^\varepsilon }\)  stands for the
natural scalar product in the Lebesgue space \(L^2(\Pi ^\varepsilon )\) .
